1. Field of the Invention
The present invention relates to wireless integrated circuit (IC) devices including wireless ICs and radiation plates. More particularly, the present invention relates to a wireless IC device used in a radio frequency identification (RFID) system.
2. Description of the Related Art
RFID systems have been developed as article management systems in recent years. In such a RFID system, a reader-writer that generates an induced magnetic field communicates with an IC tag (hereinafter referred to as a wireless IC device) that is attached to an article and that stores information by a non-contact method using the electromagnetic field to transfer information.
The wireless IC device used in an RFID system includes a wireless IC chip arranged to process a specific radio signal and a radiation plate arranged to transmit and receive the radio signal. For example, a known wireless IC device is described in WO 2007/083574.
The wireless IC device described in WO 2007/083574 includes a wireless IC chip, a power-supply circuit board on which the wireless IC chip is mounted and which includes a power supply circuit including a resonant circuit having a desired resonant frequency, and a radiation plate which is adhered to a bottom surface of the power-supply circuit board and which radiates a transmission signal supplied from the power supply circuit and receives a reception signal to supply the received reception signal to the power supply circuit. The resonant frequency of the resonant circuit in the power-supply circuit board is designed so as to substantially correspond to the frequency of the transmission and reception signals, such that the wireless IC device has very stable frequency characteristics.
Since the frequency of the radio signal transmitted and received by the radiation plate is substantially determined by the power supply circuit in the power-supply circuit board in the wireless IC device described in WO 2007/083574, the wireless IC device has very good characteristics in that the frequency of the radio signal does not significantly depend on the size and/or shape of the radiation plate. However, for example, as described in Paragraph [0020] in WO 2007/083574, the magnitude of the gain of the radio signal depends on the size and/or shape of the radiation plate. In other words, the gain varies depending on the size and/or shape of the radiation plate. However, a satisfactory configuration for successfully controlling the gain is not disclosed in WO 2007/083574.